Method of forming fixed images using heated belt

ABSTRACT

A method of forming fixed images comprising charging a photoconductor, exposing the photoconductor to light, developing a latent electrostatic image whereby a toner is applied to the latent electrostatic image formed on the photoconductor to form a visible image, transferring the formed visible image to a recording medium, and fixing the transferred visible image onto the recording medium, wherein the fixing process comprises preheating the toner transferred onto the recording medium using an endless heating film, and pressure fixing the toner.

TECHNICAL FIELD

The present invention relates to a method of forming fixed images usedfor plain paper copying machines, laser printers, plain paperfacsimiles, etc. More particularly, it relates to a method of formingimages in which low temperature fixing is carried out using a thermallydissociating encapsulated toner.

BACKGROUND ART

Conventionally, when images are formed with copying machines, laser beamprinters, etc., the Carlson Method has been generally used (U.S. Pat.Nos. 2,221,776, 2,297,691 and 2,357,809, "Electrophotography," p22-p41,R. M. Shaffert, 1965, The Focal Press).

FIG. 2 shows a schematic view of an apparatus for a conventional methodof forming fixed images. In the conventional method, after theelectrostatic latent image formed on a photoconductor by optical meansis developed in a developing process, it is transferred to a recordingmedium such as recording paper in a transfer process and then fixed intothe final image generally with heat and pressure in a fixing process. Asthe photoconductor is repeatedly used, a cleaning device is provided forcleaning the residual toner after the transfer process with itsrotation.

In the conventional method of forming fixed images, however, through theprocesses from the formation of the electrostatic latent image up to itsfixing onto the recording medium, the temperature of the heating elementof the fixing device has to remain at a very high level (usually around200° C.) and further a relatively high nip pressure is required (usuallybetween 2.0 and 6.0 kg/cm). On the other hand, since both thephotoconductor and the developing device have to be maintained at aroundroom temperature, a considerable distance has to be maintained betweenthe fixing device and the developing device, which necessitates to makethe machine larger. In addition, it is necessary to force the removal ofthe,generated heat from the system, but the noise produced by the forcedradiation device is not negligible.

Further, in the conventional method of forming fixed images, since thefixing section works independently and at such a high temperature ofaround 200° C., as mentioned above, expensive heat-resistant materialssuch as heat-resistant resins, heat-resistant rubbers, etc. have to beprovided around the fixing device.

When the fixing is carried out at a high temperature, it is subject toproblems such as curling and jamming of the paper, etc. In addition, itis pointed out that a fixing failure may take place due to the heatabsorbed by the paper, depending upon its thickness. Further, if thefixing requires a high temperature, it takes more time to reach the settemperature so that a quick printing becomes impossible. In such a case,therefore, this method is unsuitable for devices such as a facsimilewhich requires quick printings.

As for solving these problems, a device for carrying out low temperaturefixing using a cold pressing method (Japanese Patent Laid-Open No.159174/1984) is known. In this reference, however, although the fixingtemperature is low, the nip pressure has to be elevated normally to notless than 4 kg/cm in this method, making the machine heavier. Moreover,it poses problems in the gloss of the images, deformation of the papercopy sheets and an insufficient fixing strength. As for a fixing devicefor fixing images at such a low nip pressure of less than 4 kg/cm, aheat roller method is known, for example, but it has been pointed outthat the fixing temperature needs to be maintained at not less than 120°C.

Under the circumstances, the development of a fixing device that can fiximages at a low temperature and at a low nip pressure is highly desired,but it has not yet been developed. Further, as regards toners to beindispensably used for the image formation, since they have beenconfined to those made from a thermoplastic resin dispersed withadditives such as coloring agents, charge control agents, releasingagents, etc., and pulverized, there have been limitations on themolecular weight, the softening point of the thermoplastic resin for usein the toner from the aspect of storage stability, thereby posinglimitations on the further pursuit of low temperature fixing.

From these standpoints, the development of a novel method of formingfixed images as well as a matching toner thereto is in demand.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a novel method offorming fixed images, wherein an extremely low fixing temperature aswell as a low nip pressure is utilized so that the radiator can be mademuch smaller and the noise substantially reduced, thus providingadvantageous results such as the reduction of curling and jamming of thepaper sheets and quick printing.

Therefore, in view of solving the above-mentioned problems, the presentinventors have investigated a toner shell material which is fragile toheat at a low temperature. As a result, they have found that a thermallydissociating encapsulated toner produced by interfacial polymerizationmelts at a temperature of not more than 120° C., and they have furtherinvestigated the image formation method using this encapsulated tonerand have thus developed the present invention.

More particularly, the method of forming fixed images of the presentinvention comprises charging a photoconductor; exposing thephotoconductor to light; developing an electrostatic latent imagewhereby a toner is applied to the electrostatic latent image formed onthe photoconductor to form a visible image; transferring the formedvisible image to the recording medium; and fixing the transferredvisible image onto the recording medium, wherein the fixing processcomprises preheating the toner transferred onto the recording mediumusing an endless heating film and then pressure-fixing the toner.

In addition, it is a method of forming fixed images, wherein a toner isa thermally dissociating encapsulated toner, and wherein the preheatingtemperature of the toner transferred onto the recording medium is 40° C.to 120° C., and wherein the nip pressure in the fixing process is 0.5 to4 kg/cm.

According to the present invention, in the fixing process, since thepressure-fixing is carried out after preheating the toner, fixing cantake place at a low temperature. Therefore, the fixing device can besimplified, making it possible to miniaturize the fixing device andlower the cost thereof. Further, since the fixing is carried out at afixing temperature of not more than 120° C., it is not required to useheat-resistant members for the fixing device and the periphery thereof,and inexpensive materials can be used therefor. In addition, thedurability of the members used becomes long, which makes it possible toproduce a low-cost printing device. Moreover, since the fixingtemperature is very low, problems such as the curling and the jamming ofthe paper sheets are less likely to occur, and thus conserving in itsmaintenance. Further, since the toner capable of fixing at a lowtemperature is used, the temperature of the heating elements arranged inthe fixing device can be set at a lower temperature than the case wherethe ordinary toner is used. Therefore, a forced radiation device such asan electric fan, etc. can be made smaller or is not required, therebymaking it possible to reduce the noise problems. Further, the low fixingtemperature reduces the time before the set temperature is reached,making quick printing possible. The low nip pressure reduces low lineresolution and blur of the print image, thereby providing a high qualityimage and further making the durability of the fixing roller longer.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus, are notlimitative of the present invention, and wherein:

FIG. 1 is a schematic view of an apparatus used in the method of formingfixed images as defined by the present invention;

FIG. 2 is a schematic view of an apparatus used for conventional methodsof forming fixed images;

FIG. 3 is a schematic view showing the charging process in the method asdefined by the present invention;

FIG. 4 is a schematic view showing the exposing process in the method asdefined by the present invention;

FIG. 5 is a schematic view showing the developing process in the methodas defined by the present invention;

FIG. 6 is a schematic view showing the transfer process in the method asdefined by the present invention;

FIG. 7 is a schematic view of the fixing process in the method asdefined by the present invention;

FIG. 8 is a schematic view showing a cross section of the endlessheating film used in the fixing process in the method as defined by thepresent invention;

FIG. 9 is a schematic view of the fixing process in the method asdefined by the present invention; and

FIG. 10 is a schematic view of the fixing process in the method asdefined by the present invention.

The reference numerals in FIGS. 1 through 10 denote the followingelements: Element 1 is a photoconductor, element 1a a photoconductivelayer, element 1b a conductive supporter, element 2 an exposure device,element 3 a developer device, element 3a a rotating sleeve, element 5b atransfer device, element 6 a recording medium (a recording paper),element 7 a charger, element 8 a cleaner device, element 8a a tonercollecting box, element 8b a cleaning web, element 9 a charge eraser,element 10 a toner, element 11 an endless heating film, element 11a aconductive layer, element 11b a heating layer, element 11c a conductivelayer, element 11d an insulating releasing layer, element 11e aterminal, element 11f a terminal, element 12a a fixing roller (apressure roller), element 12b a fixing roller (a pressure roller),element 14 a heat roller, element 15 a conveyor belt, and element 16 aholding roller.

BEST MODE FOR CARRYING OUT THE INVENTION

The toner used in the present invention is a thermally dissociatingencapsulated toner. The encapsulated toner according to the presentinvention comprises a heat-fusible core containing at least a coloringagent and a shell formed thereon so as to cover the surface of the corematerial. In the present invention, the thermally dissociatingencapsulated toner means a toner which comprises a shell whose structureis fragile to heat, and a core material which can be fixed at a lowtemperature by pressure. More particularly, the shell structure changeswith heat, and at the point where pressure is applied, the core materialis discharged to effect the fixing of the toner. Depending on the rawmaterials and production method, a large variety of encapsulated tonersare conceivable, and as long as they are within the range of therequired thermal properties, there are no limitations on what productionprocess or materials are used. The toner in the present invention is athermally dissociating encapsulated toner, and any toner whose fixingtemperature is maintained in the range of 40° to 120° C. to therecording medium such as a recording paper can be properly chosen.

As to the method for producing the encapsulated toners, the followingcan be mentioned.

(1) Spray-drying method

After the core material is dispersed in a non-aqueous solution ofpolymer or polymer-emulsion, the dispersed liquid is spray-dried.

(2) Phase separation method (coacervation method)

In a solution of ionic polymer colloids and the core material, phaseseparation is conducted around the core material. In other words, asimple emulsion is first prepared, which in turn is converted to acomplex emulsion, in which the core materials are micro-encapsulated.

(3) Interfacial polymerization method

A core material solution or dispersion is dispersed in a water in oil oroil in water type emulsion system, while at the same time shell materialmonomers (A) are collected around the surfaces, where in the nextmethod, monomers (A) and monomers (B) react.

(4) Other methods include an in-situ polymerization method, a submergedcure coating method, an air suspension coating method, an electrostaticcoalescing method, a vacuum vapor deposition coating method, etc.

The particularly preferred toners include those produced by theinterfacial polymerization method and the spray-drying method. While thespray-drying method has the merits of an easy function separation forthe core material and shell material and a large choice of shellmaterials, the interfacial polymerization method not only has the meritof an easy function separation for the core material and shell materialbut also is capable of producing a uniform toner in an aqueous state.Moreover, substances of low softening points can be used for the corematerial in the interfacial polymerization method, making itparticularly suitable from the aspect of fixing ability. Accordingly, inthe present invention, the thermally dissociating encapsulated tonerproduced by the interfacial polymerization method among others isparticularly preferred.

For shell materials, styrene resins (Japanese Patent Laid-OpenNo.205162/1983), polyamide resins (Japanese Patent Laid-OpenNo.66948/1983), epoxy resins (Japanese Patent Laid-Open No.148066/1984),polyurethane resins (Japanese Patent Laid-Open No.179860/1982), polyurearesins (Japanese Patent Laid-Open No.150262/1987) and many others havebeen proposed. And as substances fixible under heat and pressurecontained in the core material, thermoplastic resins such as polyesterresins, polyamide resins, polyester-polyamide resins, and vinyl resinshaving glass transition points (Tg) between 10° C. and 50° C. can beused.

As compared to the thermal properties of the core material, thestructure and the thermal properties of the shell material concernthemselves remarkably with the fixing ability of the entire toner. Sincea particular polyurethane resin among the above-mentioned resins for theshell materials is thermally dissociating, having excellent storagestability and fixing ability at a low temperature, it is an extremelyfavorable material for the method of forming fixed images of the presentinvention. As principal components of such a shell material, resinsobtainable from the reaction between an isocyanate compound and/orisothiocyanate compound and compounds containing a phenolic hydroxygroup and/or a thiol group are preferably used (EP0453857A).

The thermally dissociating encapsulated toner suitably used in thepresent invention can be produced by any known methods such asinterfacial polymerization, etc., and this encapsulated toner iscomposed of a heat-fusible core material containing at least a coloringagent and a shell formed thereon so as to cover the surface of the corematerial, wherein the main components of the shell are a resin preparedby reacting:

(A) an isocyanate and/or isothiocyanate compound comprising:

(1) 0 to 30 mol % of a monovalent isocyanate and/or isothiocyanatecompounds, and

(2) 100 to 70 mol % of at least a divalent isocyanate and/orisothiocyanate compounds with

(B) an active hydrogen compound comprising:

(3) 0 to 30 mol % of a compound having one active hydrogen atom reactivewith the isocyanate and/or isothiocyanate groups and

(4) 100 to 70 mol % of a compound having at least two active hydrogenatoms reactive with the isocyanate and/or isothiocyanate groups

at a molar ratio of the component (A) to the component (B) of between1:1 and 1:20, and wherein at least 30% of all of the linkages formedfrom the isocyanate or isothiocyanate groups are thermally dissociatinglinkages.

According to the present invention, the thermally dissociating linkageis preferably one formed by the reaction between a phenolic hydroxyland/or thiol group and an isocyanate and/or isothiocyanate group.

Examples of the monovalent isocyanate compounds to be used as the thecomponent (1) in the present invention include ethyl isocyanate, octylisocyanate, 2-chloroethyl isocyanate, chlorosulfonyl isocyanate,cyclohexyl isocyanate, n-dodecyl isocyanate, butyl isocyanate, n-hexylisocyanate, lauryl isocyanate, phenyl isocyanate, m-chlorophenylisocyanate, 4-chlorophenyl isocyanate, p-cyanophenyl isocyanate,3,4-dichlorophenyl isocyanate, o-tolyl isocyanate, m-tolyl isocyanate,p-tolyl isocyanate, p-toluenesulfonyl isocyanate, 1-naphthyl isocyanate,o-nitrophenyl isocyanate, m-nitrophenyl isocyanate, p-nitrophenylisocyanate, p-bromophenyl isocyanate, o-methoxyphenyl isocyanate,m-methoxyphenyl isocyanate, p-methoxyphenyl isocyanate, ethylisocyanatoacetate, butyl isocyanatoacetate and trichloroacetylisocyanate.

Examples of the divalent or higher isocyanate compounds to be used asthe component (2) in the present invention include aromatic isocyanatecompounds such as 2,4-tolylene diisocyanate, 2,4-tolylene diisocyanatedimer, 2,6-tolylene diisocyanate, p-xylylene diisocyanate, m-xylylenediisocyanate, 4,4'-diphenylmethane diisocyanate, 1,5-naphthylenediisocyanate, 3,3'-dimethyldiphenyl-4,4'-diisocyanate,3,3'-dimethyldiphenylmethane-4,4'-diisocyanate, m-phenylenediisocyanate, triphenylmethane triisocyanate and polymethylenephenylisocyanate; aliphatic isocyanate compounds such as hexamethylenediisocyanate, trimethylhexamethylene diisocyanate, lysine diisocyanateand dimer acid diisocyanates; alicyclic isocyanate compounds such asisophorone diisocyanate, 4,4'-methylenebis(cyclohexyl isocyanate),methylcyclohexane-2,4(or 2,6)-diisocyanate and1,3-(isocyanatomethyl)cyclohexane; and other isocyanate compounds suchas an adduct of 1 mol of trimethylolpropane with 3 mol of tolylenediisocyanate.

Examples of the isothiocyanate compounds include phenyl isothiocyanate,xylylene-1,4-diisothiocyanate and ethylidene diisothiocyanate.

Among these isocyanate and isothiocyanate compounds, compounds having anisocyanate group directly bonded to an aromatic ring are preferred,because they are effective in forming a urethane resin having a lowthermal dissociation temperature.

According to the present invention, the monovalent isocyanate and/orisothiocyanate compound (1) also serves as a molecular weight modifierfor the shell-forming resin and can be used in an amount of at most 30mol % based on the isocyanate component and/or the isothiocyanatecomponent. When the amount exceeds 30 mol %, the storage stability ofthe obtained encapsulated toner is undesirably poor.

Examples of compounds having one active hydrogen atom reactive withisocyanate and/or isothiocyanate groups to be used as component (3) inthe present invention include aliphatic alcohols such as methyl alcohol,ethyl alcohol, propyl alcohol, isopropyl alcohol, butyl alcohol,isobutyl alcohol, tert-butyl alcohol, pentyl alcohol, hexyl alcohol,cyclohexyl alcohol, heptyl alcohol, octyl alcohol, nonyl alcohol, decylalcohol, lauryl alcohol and stearyl alcohol; aromatic alcohols such asphenol, o-cresol, m-cresol, p-cresol, 4-butylphenol, 2-sec-butylphenol,2-tert-butylphenol, 3-tert-butylphenol, 4-tert-butylphenol, nonylphenol,isononylphenol, 2-propenylphenol, 3-propenylphenol, 4-propenylphenol,2-methoxyphenol, 3-methoxyphenol, 4-methoxyphenol, 3-acetylphenol,4-carbomethoxyphenol, 2-chlorophenol, 3-chlorophenol, 4-chlorophenol,2-bromophenol, 3-bromophenol, 4-bromophenol, benzyl alcohol, 1-naphthol,2-naphthol and 2-acetyl-1-naphthol; and amides such as ε-caprolactam.

Particularly, a phenol derivative represented by the following formula(I) is preferably used: ##STR1## wherein R₁, R₂, R₃, R₄ and R₅ eachindependently represents a hydrogen atom, an alkyl group having 1 to 9carbon atoms, an alkenyl, alkoxy, alkanoyl, carboalkoxy or aryl group ora halogen atom.

Examples of the dihydric or higher alcohols among the compounds havingat least two active hydrogen atoms reactive with isocyanate and/orisothiocyanate groups to be used as the component (4) in the presentinvention include catechol, resorcinol, hydroquinone, 4-methylcatechol,4-tert-butylcatechol, 4-acetylcatechol, 3-methoxycatechol,4-phenylcatechol, 4-methylresorcinol, 4-ethylresorcinol,4-tert-butylresorcinol, 4-hexylresorcinol, 4-chlororesorcinol,4-benzylresorcinol, 4-acetylresorcinol, 4-carbomethoxyresorcinol,2-methylresorcinol, 5-methylresorcinol, tert-butylhydroquinone,2,5-di-tert-butylhydroquinone, 2,5-di-tert-amylhydroquinone,tetramethylhydroquinone, tetrachlorohydroquinone,methylcarboaminohydroquinone, methylureidohydroquinone,benzonorbornene-3,6-diol, bisphenol A, bisphenol S,3,3'-dichlorobisphenol S, 2,2'-dihydroxybenzophenone,2,4-dihydroxybenzophenone, 4,4'-dihydroxybenzophenone,2,2'-dihydroxydiphenyl, 4,4'-dihydroxydiphenyl,2,2'-dihydroxydiphenylmethane, 3,4-bis(p-hydroxyphenyl)hexane,1,4-bis(2-(p-hydroxyphenyl)propyl)benzene,bis(4-hydroxyphenyl)methylamine, 1,3-dihydroxynaphthalene,1,4-dihydroxynaphthalene, 1,5-dihydroxynaphthalene,2,6-dihydroxynaphthalene, 1,5-dihydroxyanthraquinone, 2-hydroxybenzylalcohol, 4-hydroxybenzyl alcohol, 2-hydroxy-3,5-di-tert-butylbenzylalcohol, 4-hydroxy-3,5-di-tert-butylbenzyl alcohol, 4-hydroxyphenethylalcohol, 2-hydroxyethyl 4-hydroxybenzoate, 2-hydroxyethyl4-hydroxyphenylacetate, resorcinol mono-2-hydroxyethyl ether,hydroxyhydroquinone, gallic acid and ethyl 3,4,5-trihydroxybenzoate.

Among these dihydric or higher alcohols, catechol derivativesrepresented by the following formula (II) and resorcinol derivativesrepresented by the following formula (III) are preferably used: ##STR2##wherein R₆, R₇, R₈ and R₉ each independently represents a hydrogen atom,an alkyl group having 1 to 6 carbon atoms, an alkenyl, alkoxy, alkanoyl,carboalkoxy or aryl group or a halogen atom. ##STR3## wherein R₁₀, R₁₁,R₁₂ and R₁₃ each independently represents a hydrogen atom, an alkylgroup having 1 to 6 carbon atoms, an alkenyl, alkoxy, alkanoyl,carboalkoxy or aryl group or a halogen atom.

Further, examples of the compounds having at least one isocyanate- orisothiocyanate-reactive functional group other than the hydroxyl groupand at least one phenolic hydroxyl group include o-hydroxybenzoic acid,m-hydroxybenzoic acid, p-hydroxybenzoic acid, 5-bromo-2-hydroxybenzoicacid, 3-chloro-4-hydroxybenzoic acid, 4-chloro-2-hydroxybenzoic acid,5-chloro-2-hydroxybenzoic acid, 3,5-dichloro-4-hydroxybenzoic acid,3-methyl-2-hydroxybenzoic acid, 5-methoxy-2-hydroxybenzoic acid,3,5-di-tert-butyl-4-hydroxybenzoic acid, 4-amino-2-hydroxybenzoic acid,5-amino-2-hydroxybenzoic acid, 2,5-dinitrosalicylic acid, sulfosalicylicacid, 4-hydroxy-3-methoxyphenylacetic acid, catechol-4-carboxylic acid,2,4-dihydroxybenzoic acid, 2,5-dihydroxybenzoic acid,2,6-dihydroxybenzoic acid, 3,4-dihydroxybenzoic acid,3,5-dihydroxybenzoic acid, 3,4-dihydroxyphenylacetic acid,m-hydroxycinnamic acid, p-hydroxycinnamic acid, 2-amino-4-methylphenol,2-amino-5-methylphenol, 5-amino-2-methylphenol, 3-amino-2-naphthol,8-amino-2-naphthol, 1-amino-2-naphthol-4-sulfonic acid,2-amino-5-naphthol-4-sulfonic acid, 2-amino-4-nitrophenol,4-amino-2-nitrophenol, 4-amino-2,6-dichlorophenol, o-aminophenol,m-aminophenol, p-aminophenol, 4-chloro-2-aminophenol,1-amino-4-hydroxyanthraquinone, 5-chloro-2-hydroxyaniline,α-cyano-3-hydroxycinnamic acid, α-cyano-4-hydroxycinnamic acid,1-hydroxynaphthoic acid, 2-hydroxynaphthoic acid, 3-hydroxynaphthoicacid and 4-hydroxyphthalic acid.

Further, examples of the polythiol compounds having at least one thiolgroup in each molecule include ethanethiol, 1-propanethiol,2-propanethiol, thiophenol, bis(2-mercaptoethyl)ether,1,2-ethanedithiol, 1,4-butanedithiol, bis(2-mercaptoethyl) sulfide,ethylene glycol bis(2-mercaptoacetate), ethylene glycolbis(3-mercaptopropionate), 2,2-dimethylpropanediolbis(2-mercaptoacetate), 2,2-dimethylpropanediolbis(3-mercaptopropionate), trimethylolpropane tris(2-mercaptoacetate),trimethylolpropane tris(3-mercaptopropionate), trimethylolethanetris(2-mercaptoacetate), trimethylolethane tris(3-mercaptopropionate),pentaerythritol tetrakis(2-mercaptoacetate), pentaerythritoltetrakis(3-mercaptopropionate), dipentaerythritolhexakis(2-mercaptoacetate), dipentaerythritolhexakis(3-mercaptopropionate), 1,2-dimercaptobenzene,4-methyl-1,2-dimercaptobenzene, 3,6-dichloro-1,2-dimercaptobenzene,3,4,5,6-tetrachloro-1,2-dimercaptobenzene, xylylenedithiol and1,3,5-tris(3-mercaptopropyl) isocyanurate.

In the thermally dissociating shell-forming resin used in the presentinvention, at least 30%, preferably at least 50% of all of the linkagesformed from isocyanate or isothiocyanate groups are thermallydissociating linkages. When the content of the thermally dissociatinglinkages is less than 30%, the strength of the shell in theheat-and-pressure fixing cannot be sufficiently lowered, making it lesslikely to exhibit any advantageous fixing performance of the corematerial.

In the thermally dissociating encapsulated toner of the presentinvention, other compounds having an isocyanate-reactive functionalgroup other than phenolic hydroxyl and thiol groups, which may be usedas a shell-forming material in such an amount as not to lower the ratioof the linkages formed by the reaction of isocyanate and/orisothiocyanate groups with phenolic hydroxyl and/or thiol groups to theall of the linkages formed from isocyanate and/or isothiocyanate groupsis less than 30%, include, for example, the following active methylenecompounds such as malonate and acetoacetate, oxime such as methyl ethylketone oxime, carboxylic acid, polyol, polyamine, aminocarboxylic acidand aminoalcohol.

According to the present invention, the compound having one activehydrogen atom reactive with isocyanate and/or isothiocyanate groups asthe component (3) may be used in an amount of at most 30 mol % based onthe active hydrogen component. When the amount exceeds 30 mol %, thestorage stability of the resulting toner is undesirably poor.

Further, the molar ratio of (A) the isocyanate compound and/orisothiocyanate compound comprising the components (1) and (2) to (B) theactive hydrogen compounds comprising the components (3) and (4)preferably lies between 1:1 and 1:20 in order to obtain a resin freefrom unreacted isocyanate groups.

In the production of the encapsulated toner according to the presentinvention, the shell is preferably formed by an interfacialpolymerization or an in-situ polymerization. Alternatively, it may beformed by a dry method comprising stirring in an air stream at a highrate matrix particles used as a core material together with particlesused as a shell-forming material having a number-average particle sizeof one-eighth or less of that of the matrix particles.

The resins to be used as the shell materials can be produced in thepresence of no catalysts; however, when the resins are produced in thepresence of catalysts, those catalysts including tin catalysts such asdibutyltindilaurate, etc., amine catalysts such as1,4-diazabicyclo[2.2.2]octane,N,N,N-tris-(dimethylaminopropyl)-hexahydro-S-triazine, etc. and anyknown urethane catalysts can be used.

The resins to be used as core materials of the encapsulated toneraccording to the present invention are thermoplastic resins having glasstransition points (Tg) of 10° to 50° C., and examples thereof includepolyester resins, polyester-polyamide resins, polyamide resins and vinylresins, among which vinyl resins are particularly preferable. When theglass transition point (Tg) is less than 10° C., the storage stabilityof the resulting encapsulated toner is undesirably poor, and when itexceeds 50° C., the fixing strength of the encapsulated toner isundesirably poor.

Examples of the monomers constituting the vinyl resins include styreneand its derivatives such as styrene, o-methylstyrene, m-methylstyrene,p-methylstyrene, α-methylstyrene, p-ethylstyrene, 2,4-dimethylstyrene,p-chlorostyrene and vinylnaphthalene; ethylenically unsaturatedmonoolefins such as ethylene, propylene, butylene and isobutylene; vinylesters such as vinyl chloride, vinyl bromide, vinyl fluoride, vinylacetate, vinyl propionate, vinyl formate and vinyl caproate; ethylenicmonocarboxylic acids and esters thereof such as acrylic acid, methylacrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butylacrylate, isobutyl acrylate, tert-butyl acrylate, amyl acrylate,cyclohexyl acrylate, n-octyl acrylate, isooctyl acrylate, decylacrylate, lauryl acrylate, 2-ethylhexyl acrylate, stearyl acrylate,methoxyethyl acrylate, 2-hydroxyethyl-acrylate, glycidyl acrylate,2-chloroethyl acrylate, phenyl acrylate, methyl α-chloroacrylate,methacrylic acid, methyl methacrylate, ethyl methacrylate, n-propylmethacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutylmethacrylate, tert-butyl methacrylate, amyl methacrylate, cyclohexylmethacrylate, n-octyl methacrylate, isooctyl methacrylate, decylmethacrylate, lauryl methacrylate, 2-ethylhexyl methacrylate, stearylmethacrylate, methoxyethyl methacrylate, 2-hydroxyethyl methacrylate,glycidyl methacrylate, phenyl methacrylate, dimethylaminoethylmethacrylate and diethylaminoethyl methacrylate; ethylenicmonocarboxylic acid derivatives such as acrylonitrile, methacrylonitrileand acrylamide; ethylenic dicarboxylic acids and derivatives thereofsuch as dimethyl maleate; vinyl ketones such as vinyl methyl ketone;vinyl ethers such as vinyl methyl ether; vinylidene halides such asvinylidene chloride; and N-vinyl compounds such as N-vinylpyrrole andN-vinylpyrrolidone.

Among the above core material resin-constituting monomers according tothe present invention, the core material-forming resin contains, in themain skeleton of the resin, styrene or its derivatives preferably in anamount of 50 to 90 parts by weight, and the ethylenic monocarboxylicacid or an ester thereof preferably in an amount of 10 to 50 parts byweight to control the thermal properties of the resin, such as thesoftening point.

When the monomer composition constituting the core material-formingresin according to the present invention contains a crosslinking agent,which may be also used, if necessary, as a mixture of two or more ofthem, any known crosslinking agents may be properly used. When theamount of the crosslinking agent added is too large, the resulting toneris less likely to be heat-fused, thereby resulting in poor heat fixingability and heat-and-pressure fixing ability. On the contrary, when theamount is too small, in heat-and-pressure fixing, a part of the tonercannot be completely fixed on a paper but rather adheres to the surfaceof a roller, which in turn is transferred to a subsequent paper whichcreates the so-called "offset" or "offset phenomenon." Accordingly, theamount of the crosslinking agent is preferably 0.001 to 15% by weight,more preferably 0.1 to 10% by weight, based on the monomers used.

The core material of the thermally dissociating encapsulated toneraccording to the present invention may further contain, if necessary,one or more offset inhibitors of any known kind for the purpose ofimproving offset resistance at heat-and-pressure fixing. These offsetinhibitors are contained in an amount of 1 to 20% by weight based on theresin contained in the core material.

The core material of the thermally dissociating encapsulated toneraccording to the present invention contains a coloring agent, which maybe any one of the dyes and pigments used in the conventional toners. Thecoloring agent is generally contained in an amount of 1 to 15 parts byweight per 100 parts by weight of the resin contained in the corematerial.

In addition, in the shell-forming materials of the thermallydissociating encapsulated toner according to the present invention andin the core material, a metal-containing dye which has been used fortoners, for example, a metal complex of an organic compound having acarboxyl or nitrogenous group, such as nigrosine, may be added in aneffective amount as a charge control agent. Alternatively, such a chargecontrol agent may be mixed with the toner.

The thermally dissociating encapsulated toner according to the presentinvention may contain, if necessary, a fluidity improver and/or acleanability improver. Further, for the purpose of controlling thedevelopability of the encapsulated toner, an additive, for example,finely powdered polymethyl methacrylate, etc. may be added. Furthermore,for the purposes of toning or resistance control, a small amount ofcarbon black may be used.

The thermally dissociating encapsulated toner of the present inventionpreferably has a softening point of not less than 80° C. and not morethan 150° C. If the softening point is lower than 80° C., the offsetresistance of the resulting encapsulated toner is undesirably poor, andwhen it exceeds 150° C., the fixing strength of the encapsulated toneris undesirably poor.

Although the particle size of the encapsulated toner according to thepresent invention is not particularly limited, the average particle sizethereof is generally 3 to 30 μm. The preferred thickness of the shell ofthe encapsulated toner is from 0.01 to 1 μm. When the thickness is lessthan 0.01 μm, the blocking resistance of the resulting encapsulatedtoner is poor, and when it exceeds 1 μm, the heat fusibility of theresulting encapsulated toner is undesirably poor.

Examples of the thermally dissociating encapsulated toners which ispreferably used in the present invention are described above, but thepresent invention is not confined to these alone.

The method of forming fixed images of the present invention are detailedbelow, referring to the drawings. FIG. 1 is a schematic view of anapparatus used for the method of forming fixed images as defined by thepresent invention. Element 1 is a photoconductor such as of amorphoussilicon or organic photoconductor, etc. in which a photoconductive layeris provided on a conductive supporter. For photoconductors, thosepractically used are photoconductors of selenium, silicon, organicgroups, etc., and any of these can be used. Element 7 is a chargerlocated opposite to the photoconductor 1. The charging means is notparticularly restricted, and any of, for example, a corona charger, abrush charger, a roller charger, etc. can be used. Element 2 is anexposure device located opposite to the photoconductor 1 for formingelectrostatic latent images on the photoconductor surface. For anexposure device 2, light sources such as laser beams, LED or EL Arrays,etc. are used in combination with an image-forming optical system.Alternatively, a device based on optical systems projecting a reflectedlight of a document usually provided in the copying machine can be used.Element 3 is a developer device located opposite to the photoconductor 1for making visible the electrostatic latent image formed on thephotoconductor with the toner. For a developer device, any of thecommonly used two-component magnetic brush developer devices, theone-component magnetic brush developer device, and the one-componentnon-magnetic developer device, etc. can be used. The toners to be usedin the present invention are thermally dissociating encapsulated toners,which are produced by an interfacial polymerization method, etc.

The toner applied to the electrostatic latent image formed on thephotoconductor is transferred by the transfer device 5b to the recordingmedium 6. Known as transfer methods are a corona transfer method,wherein corona ions are supplied to the reverse side of the recordingmedium; a roller transfer method, wherein a transfer electric field isformed by voltage generated by pressing a conductive roller, to which avoltage is applied, against the reverse side of the recording medium;and an induction belt transfer method, wherein an inductive belt servesto convey the recording medium, etc., and all of these methods areapplicable to the present invention.

The cleaner device 8 such as a cleaning web for removing trace amountsof the toner remaining on the photoconductor after the transfer processis placed opposite to the photoconductor 1.

In the fixing process, the toner transferred onto the recording mediumis first preheated by an endless heating film. As long as the heatingfilm can heat the toner on the recording medium to a temperature of upto 120° C., there are no limitations on the materials used therefor. Forinstance, those generally known as heating sheets can be used. Examplesinclude those obtained by dispersing a conductive material such asconductive carbon, conductive inorganic powders, conductive whiskers ofpotassium titanate, etc. into a resin such as a polyamide resin, apolyamide-imide resin, a polycarbonate resin, etc., and thus moldinginto a formed body (Japanese Patent Laid-Open Nos. 10592/1977,23740/1977 and 5795/1978).

In addition, those obtained by producing a circuit pattern on aninsulating releasing layer using metal foils of aluminum, nickel,stainless steel, nichrome, etc. as a resistance heating element by suchmethods as photoetching, pressing, printing, etc. so as to meet specificrequirements for temperature characteristics can be used (JapanesePatent Laid-Open Nos. 90243/1973, 90225/1973 and 115343/1974).

Among those mentioned above, those blends of the conductive materials orthose capable of freely adjusting the heating temperature by a circuitpattern and thus generating far infrared radiation are preferred heatingelements for an endless heating film in the present invention, becausethey have remarkably good heating efficiency, small energy consumptionand fast temperature-rising speed. Examples thereof include, forinstance, a heating element described in Japanese Patent Laid-Open No.19889/1981. In addition, those having PTC characteristics (PositiveThermo Conductivity: As the temperature increases, the resistivityincreases.) can adjust their own temperature, thus making it morepreferred heating elements. Examples thereof include, for instance,those heating elements described in Japanese Patent Laid-Open Nos.10592/1977, 5795/1978 and 14034/1979).

FIG. 8 shows a schematic cross-sectional view of one example of aheating film. The heating film comprises a fluorine resin film (aninsulating releasing layer 11d) of 75 μm in thickness; an aluminumdeposition layer (a conductive layer 11c) formed on the fluorine resinfilm; a heating layer 11b comprising a thermoplastic resin dispersedwith conductive carbon formed on the surface of the aluminum depositionlayer; and an aluminum foil (a conductive layer 11a) formed on theheating layer 11b, whereby the heating layer 11b is laminated with theconductive layers 11a and 11c to construct a sandwich-like structure. Inaddition, heat in the heating film can be generated by connectingterminals to each of two conductive layers (11a and 11c), and conductingelectricity. The heating film can be heated quickly up to a desiredtemperature by adjusting the resistivity in the heating layer andapplied voltage. Alternatively, as shown in FIG. 9, it is also possibleto use a heating film produced by laminating the heating layer 11b andthe insulating releasing layer 11d. In this case, terminals 11e and 11fare arranged to contact with the heating layer 11b, and the effectivelength between those terminals is normally 20 to 400 mm, preferably 40to 200 mm.

As shown in FIGS. 7 and 9, the endless heating film comprising the aboveheating film is stretched with at least one pair of rollers, forexample, a fixing roller (a pressure roller) 12a and a holding roller16. In addition, upon the stretching of the endless heating film, numberof rollers can be increased, if necessary. For example, as shown in FIG.10, a stretched endless heating film is used.

The toner to be fixed onto the recording medium is conveyed in paralleland without being in contact with the endless heating film to the fixingportion comprising a pair of the fixing rollers (the pressure rollers)12a, 12b. For instance, as shown in FIGS. 1 and 7, the recording medium6 is conveyed along the surface in parallel with the endless heatingfilm 11 by a conveyor belt 15. The toner adhered onto the recordingmedium while conveying the recording medium is preheated by the heattransmitted from the endless heating film. In this case, the heatingtemperature of the endless heating film is properly adjusted based onthe conveying speed of the recording medium, so that the preheatingtemperature of the toner falls within specified temperature ranges.

As a means for pressure-fixing the toner thus preheated onto therecording medium, a pair of fixing rollers (pressure rollers) can beused. The toner is fixed by inserting the recording medium between thepair of the fixing rollers (the pressure rollers) 12a, 12b through theendless heating film. Specifically, elements 12a, 12b are fixing rollers(pressure rollers), which are arranged so that the recording medium canbe pressed through the endless heating film at a specified nip pressure.Since fixing takes place in the case of ordinary fixing devices at ahigh temperature (around 200° C.), expensive heat-resistant siliconerubber, Teflon resin, etc. must be used for the fixing rollers (pressurerollers) 12a, 12b. However, since the surface temperature of the tonerpreheated by using the endless heating film is at most 120° C. in thepresent invention, the temperature transmitted to the fixing roller isvery low. Therefore, a high heat resistance is not required for thefixing roller. Accordingly, as long as it is an elastic member having asoftening point of not less than 120° C., there are no limitations onits material, and any of the ordinary inexpensive elastic materials canbe used. For instance, polyester resins, nylon resins, heat-resistantpolyurethane resins, heat-resistant synthetic rubbers, etc. can be used.Further, since such a low nip pressure as less than 4 kg/cm isapplicable to a fixing device in the present invention, the durabilityof the fixing roller becomes longer.

A cleaning device such as a cleaning web 8b is arranged in the fixingdevice for the purpose of removing trace amounts of the toner remainingon the endless heating film after the fixing process. As long as thetoner remaining on the endless heating film can be removed, thearranging position of the cleaning web 8b is not particularly limited.For instance, as shown in FIGS. 1, 7 and 9, the cleaning web 8b may bearranged opposite to the fixing roller (the pressure roller) 12a.Alternatively, as shown in FIG. 10, it may be arranged opposite to theholding roller 16. After fixing process, the recording medium isdischarged out of the apparatus by a particular paper discharging meansnot illustrated in the figure.

The photoconductor 1, the endless heating film 11, the conveyor belt 15,the fixing rollers (the pressure rollers) 12a and 12b and the holdingroller 16 are rotated by specified driving means not illustrated in thefigures in the direction shown in the respective drawings at specifiedperipheral speeds.

In general, the toners are fixed by a conventionally known heat rollermethod. In this method, however, since the nip width cannot be takenwidely, the time during which the recording medium contacts the heatroller surface is extremely short, thereby simultaneously supplying heatand pressure in a short period of time. Accordingly, the surfacetemperature of the heat roller is required to be extremely high.However, in the present invention, the toner transferred onto therecording medium is fixed by being pressed with a fixing roller aftersufficiently heating the internal portion of the toner adhered onto therecording medium by preheating using the endless heating film.Therefore, as compared to the conventional heat roller method, fixing ofthe toner onto the recording medium can take place at an extremely lowfixing temperature and a low nip pressure. Particularly the use of thethermally dissociating encapsulated toner according to the presentinvention is highly effective, because the shell material is fragile toheat, and thus the core material having a low glass transition point canbe fixed.

Next, the individual processes of the method of forming fixed images bythe present invention having the above-mentioned construction will bedescribed.

FIG. 3 shows a charging process, FIG. 4 an exposing process, FIG. 5 adeveloping process, FIG. 6 a transfer process and FIGS. 7 through 10fixing processes.

In the charging process, as shown in FIG. 3, a specified charge isuniformly supplied, e.g. by the corona charger 7 to the photoconductorsurface. A photoconductor sensitive to a positive charge is taken herefor an example, and the surface of the conductive supporter 1b is coatedwith the photoconductive layer 1a to form the photoconductor 1. Auniform charge is applied by the corona charger 7 to the photoconductivelayer 1a, thereby positively charging the surface of the photoconductivelayer 1a.

In the exposing process, as shown in FIG. 4, a light from the exposuredevice 2 is irradiated to the surface of the above photoconductor, sothat a leakage of charges occurs only in the exposed parts to form anelectrostatic latent image on the photoconductive layer 1a.

In the developing process, as shown in FIG. 5, the tonertriboelectrically charged inside the developer device is transported bythe rotating sleeve 3a, and developed onto the photoconductor surface inproportion to the charge on the photoconductor surface. The developingprocess is an assortment of normal development in which a reverselypolarized toner adheres to the charges by the Coulomb's force and ofreverse development in which the toner adheres to the charges lost dueto exposure to the light. The development process in the presentinvention applies to either method, but the case of the normaldevelopment is illustrated in FIG. 5.

In the transfer process, as shown in FIG. 6, the visible image formed onthe photoconductor surface accepts the charges from the reverse side ofthe recording medium 6 such as the recording paper through atransfer-corotron or a transfer-roller, and it is then transferred tothe recording medium 6. Part of the toner is left behind untransferredon the photoconductor surface, which is removed by the cleaning device 8such as a cleaning web arranged opposite to the photoconductor as shownin FIG. 1.

In the fixing process, as shown in FIGS. 1 and 7, while the recordingmedium 6 is conveyed along the surface of the endless heating film 11without being in contact therewith by a means of the conveyor belt 15,the visible image transferred onto the recording medium 6 is heated inadvance by the heat transmitted from the endless heating film 11, andthen the surface of the recording medium onto which the visible image isadhered is pressure-welded to the surface of the endless heating film bya pair of the fixing rollers (the pressure rollers) 12a and 12b, therebystrongly fixing the visible image onto the recording medium 6.

In the present invention, the toner on the recording medium is heated inthe process of conveying within the temperature range of normallybetween 40° C. and 120° C., preferably between 60° C. and 120° C. Whenthe heating temperature is less than 40° C., the melting of the tonerbecomes insufficient, and when it exceeds 120° C., the fixingtemperature becomes too high, posing problems incurred by theconventional methods as mentioned above.

In the case of the conventional methods, the nip pressure in the fixingprocess has to be made higher, if the fixing temperature is made lower,thereby requiring a nip pressure of usually not less than 4 kg/cm.However, in the present invention, although the fixing temperature isset to be not more than 120° C., a sufficient fixing strength can beobtained with a nip pressure of normally 0.5 to 4 kg/cm, and even lessthan 2 kg/cm in many cases. Therefore, fixing can take place at a lownip pressure, thereby making the durability of the fixing roller longer.Moreover, in general, if the temperature applied to the surface of therecording medium is too high, the recording paper tends to curl. If thetemperature is too low, the fixing of the toner becomes insufficient,making record preservation difficult. Therefore, since the fixing can becarried out in the temperature range of 40° C. to 120° C. in the presentinvention as mentioned above, such problems are not likely to takeplace.

In addition, the toner remaining on the surface of the endless heatingfilm after fixing process is removed by cleaning means such as acleaning web 8b, so that the endless heating film can be repeatedlyused.

On the other hand, the charges remaining on the photoconductor 1 afterthe developing process and the transfer process are over are neutralizedby a charge eraser 9 such as a charge erasing lamp into a reusable stateagain for the charging process.

In addition, the present invention is not confined to theabove-mentioned embodiments, and specifications of the kinds ofindividual apparatus, processes etc. can be revised based on theprinciples of the present invention.

By using the method of forming fixed images of the present invention,the following effects can be obtained:

(1) Since the pressure-fixing is carried out after preheating the toner,fixing takes place at a low temperature.

(2) Since the fixing is carried out at a fixing temperature of not morethan 120° C., the fixing apparatus can be simplified, making it possibleto miniaturize the fixing device and lower the cost.

(3) Since the fixing is carried out at a fixing temperature of not morethan 120° C., heat-resistant members which have been required for theconventional devices are not required for the fixing device and theperiphery thereof in the present invention, making it possible to useless expensive materials and lower the cost.

(4) Since the fixing is carried out at a fixing temperature of not morethan 120° C. with a low nip pressure, paper sheets become less likely tocurl or jam, and thus conserving in its maintenance.

(5) Since the fixing is carried out at a fixing temperature of not morethan 120° C. with a lower nip pressure, durability of the components ofthe fixing device and the periphery thereof becomes longer, and thusconserving in its maintenance.

(6) Since a toner for the low-temperature fixing is used, thetemperature of the heating element in the fixing device can be set lowwith only a small rise of the temperature in the printing machine.Accordingly, a forced radiation device such as an electric fan can bemade smaller or is not required, thereby reducing the noise problem.

(7) Since the waiting time for the temperature rise in the fixing devicecan be shortened, quick printing is made possible.

The present invention is hereinafter described in more detail by meansof the following working examples, but the present invention is notlimited by them.

PRODUCTION EXAMPLE OF ENCAPSULATED TONER

To a mixture comprising 70.0 parts by weight of styrene, 30.0 parts byweight of 2-ethylhexyl acrylate and 1.0 part by weight ofdivinylbenzene, 10.0 parts by weight of carbon black "#44" (manufacturedby Mitsubishi Chemical Industries, Ltd.), 4.0 parts by weight of2,2'-azobisisobutyronitrile, 9.5 parts by weight of 4,4'-diphenylmethanediisocyanate "Millionate MT" (manufactured by Nippon PolyurethaneIndustry Co., Ltd.) are added. The obtained mixture is introduced intoan attritor (manufactured by Mitsui Miike Kakoki) and dispersed at 10°C. for 5 hours to give a polymerizable composition. This composition isadded to 800 g of a 4% by weight aqueous colloidal solution oftricalcium phosphate which had been preliminarily prepared in a 2-literseparable glass flask, so as to give a concentration of 30% by weight.The obtained mixture is emulsified and dispersed with a TK homomixer(manufactured by Tokushu Kika Kogyo) at 5° C. and a rotational speed of10000 rpm for 2 minutes. A four-necked glass cap is set on the flask,and a reflux condenser, a thermometer, a dropping funnel fitted with anitrogen inlet tube and a stainless steel stirring rod are set thereon.The resulting flask is placed on an electric mantle heater. A solutionof 22.0 g of resorcinol, 3.6 g of diethyl malonate and 0.5 g of1,4-diazabicyclo[2.2.2] octane in 40 g of ion-exchanged water isprepared, and the resulting mixture is dropped into the flask in aperiod of 30 minutes through the dropping funnel while stirring.Thereafter, the contents are heated to 80° C. and reacted for 10 hoursin a nitrogen atmosphere while stirring. After cooling the reactionmixture, it is dissolved into 10%-aqueous hydrochloric acid. Theresulting mixture is filtered and the obtained solid is washed withwater, dried under a reduced pressure of 20 mmHg at 45° C. for 12 hoursand classified with an air classifier to give the encapsulated tonerwith an average particle size of 9 μm having a shell made of a resinhaving a thermally dissociating urethane linkage. The glass transitionpoint assignable to the resin contained in the core material is 30.2°C., and its softening point is 130.0° C.

PRODUCTION EXAMPLE OF REFERENCE TONER

To 100 parts by weight of a polyester resin (Bisphenol-type polyesterresin; softening point:. 135° C.; Tg: 65° C.), 7 parts by weight ofcarbon black (manufactured by Mitsubishi Kasei Ltd., MA8), 3 parts byweight of a polypropylene wax (Sanyo Kasei Ltd., Siscol 660P), and 2parts by weight of a charge control agent (Hodogaya Kagaku Ltd.,Aizenspilon Black TRH) are mixed, and the resulting mixture is kneadedby a pressurized kneader. After sufficiently dispersing the obtainedmixture, it is pulverized with a pulverizing mill and then classifiedwith a classifier to obtain a toner having a particle distribution rangeof 5 to 25 μm and an average particle size of 10 μm. To 1 kg of thetoner, 5 g of colloidal silica (Nihon Aerozil Ltd.: R972) is externallyadded to obtain a surface-treated reference toner.

TEST EXAMPLE 1

50 g of the toner obtained in Production Example of Encapsulated Toneris blended together with 1 kg of a commercially available ferritecarrier by using a V-type blender to obtain a developer 1. The obtaineddeveloper 1 is loaded on a commercially available copying machine todevelop images without fixing. The fixing ability and the non-offsettingregion of the toner of the present invention are measured using thefixing device of the present invention schematically shown in FIG. 7(roller diameter: 20 mmφ; nip pressure: 1.0 kg/cm; effective length ofheating film: 50 mm), while varying the heating temperature at a linearvelocity of 20 mm/sec. As a result, the lowest fixing temperature is 80°C., and the non-offsetting region of the toner is at a temperaturebetween 70° C. and 180° C.

On the other hand, the toner obtained by the Production Example ofReference Toner is mixed with a commercially available ferrite carrierto prepare a developer 2. After developing images in the same manner asabove using a commercially available copying machine, the fixing abilityand the non-offsetting region of the reference toner are measured usingthe fixing device of the present invention. As a result, the lowestfixing temperature is 125° C., and the non-offsetting region of thetoner is at a temperature between 100° C. and 180° C.

TEST EXAMPLE 2

The developer 1 obtained in Test Example 1 is loaded on a commerciallyavailable copying machine to develop images without fixing. The fixingability and the non-offsetting region of the toner of the presentinvention are measured using a conventional fixing device schematicallyshown in FIG. 2 (roller diameter: 30 mmφ; nip pressure: 2.0 kg/cm; analuminum surface of the heat roller being surface-coated with Teflon ina thickness of 50 μm; the pressure roller being heat-resistant siliconerubber roll), while varying the heating temperature at a linear velocityof 20 mm/sec. As a result, the lowest fixing temperature is 100° C., andthe non-offsetting region of the toner is at a temperature between 80°C. and 200° C.

On the other hand, by using the developer 2 obtained in the Test Example1, developing images are obtained in the same manner as above using acommercially available copying machine. After that, the fixing abilityand the non-offsetting region of the reference toner are measured usingthe same fixing device as above. As a result, the lowest fixingtemperature is 135° C., and the non-offsetting region of the toner is ata temperature between 115° and 200° C.

The lowest fixing temperature for the toner is the temperature of thepaper surface at which the fixing rate of the toner exceeds 70%. Thisfixing rate of the toner is determined by placing a load of 500 g on asand-containing rubber eraser having a bottom area of 15 mm×7.5 mm whichcontacts the fixed toner image, placing the loaded eraser on a fixedtoner image obtained in the fixing device, moving the loaded eraser onthe image backward and forward five times, measuring the opticalreflective density of the eraser-treated image with a reflectivedensitometer manufactured by Macbeth Co., and then calculating thefixing rate from this density value and a density value before theeraser treatment using the following equation. ##EQU1##

From these test examples, it is confirmed that by utilizing the methodof forming fixed images according to the present invention, the lowestfixing temperature can be remarkably lowered when compared to the casewhere the conventional heat roller-type fixing device is used. This isparticularly remarkably effective in a case where a thermallydissociating encapsulated toner is used.

The present invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

We claim:
 1. A method of forming fixed images comprisingcharging aphotoconductor; exposing said photoconductor to light, thereby formingan electrostatic latent image on the surface of said photoconductor;developing said electrostatic latent image by applying an encapsulatedtoner, comprising a heat-fusible core material containing at least acoloring agent and a shell formed thereon so as to cover the surface ofthe core material, to said electrostatic latent image formed on thesurface of said photoconductor to form a visible image; transferringsaid visible image to a recording medium; and fixing the transferredvisible image onto said recording medium by a process comprisingpreheating said encapsulated toner transferred onto said recordingmedium to a temperature of 120° C. or less by conveying the recordingmedium in a direction parallel to that of an endless heating film,without contact between said recording medium and said endless heatingfilm, and then pressure-fixing the toner.
 2. The method according toclaim 1, wherein said toner is a thermally dissociating encapsulatedtoner.
 3. The method according to claim 1, wherein the preheatingtemperature of the toner transferred onto the recording medium is 40° C.to 120° C.
 4. The method according to claim 1, wherein a nip pressure inthe fixing process is 0.5 to 4 kg/cm.
 5. The method according to claim2, wherein said thermally dissociating encapsulated toner comprises aheat-fusible core material containing at least a coloring agent and ashell formed thereon so as to cover the surface of the core material,wherein the main component of the shell is a resin prepared byreacting:(A) an isocyanate and/or isothiocyanate compound comprising:(1)0 to 30 mol % of a monovalent isocyanate and/or isothiocyanatecompounds, and (2) 100 to 70 mol % of at least a divalent isocyanateand/or isothiocyanate compounds with (B) an active hydrogen compoundcomprising:(3) 0 to 30 mol % of a compound having one active hydrogenatom reactive with the isocyanate and/or isothiocyanate groups and (4)100 to 70 mol % of a compound having at least two active hydrogen atomsreactive with the isocyanate and/or isothiocyanate groupsat a molarratio of the component (A) to the component (B) of between 1:1 and 1:20,and wherein at least 30% of all of the linkages formed from theisocyanate or isothiocyanate groups are thermally dissociating linkages.6. The method according to claim 5, wherein said thermally dissociatinglinkage is a linkage derived from reacting phenolic hydroxyl and/orthiol groups with the isocyanate and/or isothiocyanate groups.
 7. Themethod according to claim 5, wherein said heat-fusible core materialcomprises a thermoplastic resin, as its main component, whose glasstransition point is 10° C. to 50° C.
 8. The method according to claim 5,wherein the softening point of said thermally dissociating encapsulatedtoner is 80° C. to 150° C.